1. Field of the Invention
The present invention relates to an optical fiber probe and, more specifically to an optical fiber probe having high heat resistance and high pressure tightness.
2. Description of the Related Art
Combustion condition in a combustor, such as a gas turbine combustor, is diagnosed on the basis of the luminance of flames measured with an optical fiber probe during combustion, and combustion is controlled on the basis of the result of diagnosis. Optical fiber probes are exposed to high temperatures in measuring the luminance of flames, and hence the optical fiber probes are cooled by forced cooling using cooling water or cooling air. Thus, water-cooled optical fiber probes and air-cooled optical fiber probes are used.
A flame luminance measuring device using a water-cooled optical fiber probe needs a cooling water circulating system for circulating cooling water through the water-cooled optical fiber probe. Therefore, the flame luminance measuring device inevitably has complicated construction and is heavy. The heaviness of the flame luminance measuring device is a fatal disadvantage of the flame luminance measuring device using a water-cooled optical fiber probe, when the luminance measuring device is applied to an aircraft gas turbine combustor. The water circulating system needs additional driving power, increases the running cost of the flame luminance measuring device, and requires troublesome maintenance work.
A flame luminance measuring device using an air-cooled optical fiber probe inevitably has problems, though not as serious as those of the flame luminance measuring device using a water-cooled optical fiber probe, arising from the intricacy of construction, large weight, high running cost and the troublesomeness of maintenance work. If air supplied from a compressor is used as cooling air, the efficiency of the gas turbine decreases.
FIG. 4 shows a heat-resistant terminal structure for an optical fiber probe proposed in JP 4-98010 U to solve problems in water-cooled and air-cooled optical fiber probes. The heat-resistant terminal structure comprises, a bare optical fiber 101, a ceramic collet 102, a protective metal pipe 103, and a tip holder 104 holding a tip part of the bare optical fiber 101 adhesively bonded thereto in the ceramic collet 103. Since the optical fiber 101 and the ceramic collet 102 have different coefficients of thermal expansion, respectively, the holder 104 is unable to hold a sufficiently long tip part of the optical fiber 101. Consequently, the heat-resistant terminal structure has insufficient pressure tightness. The heat-resistant terminal structure needs an expensive adhesive for bonding the tip part of the optical fiber 101 to the holder 104.